Herbicide deposition and wash-off potential is affected by cover crop management tactics used in planting green systems

There is increasing interest in enhancing weed-suppression potential from cover crop surface residues by delaying termination until at or after cash crop planting, often referred to as “planting green.” The combination of increased cover crop biomass production and application of soil-residual herbicides into living cover crops in planting green systems may significantly affect herbicide deposition to the soil surface and wash-off from residues. We conducted field studies to describe the (1) relative effects of cover crop management tactics on herbicide deposition to the soil surface at the time of application; and (2) the influence of cover crop management tactics on herbicide wash-off potential from living cover crops (0 d after termination [DAT]) and aged early postemergence residues. Pyroxasulfone was used as the test herbicide. Our results indicate that in scenarios with standing cereal rye (Secale cereale L.) below 2 Mg ha⁻¹, herbicide deposition is reduced by approximately 35% relative to bare ground regardless of application timing (0 DAT, early postemergence). At 5 Mg ha⁻¹, herbicide deposition is reduced by 50% regardless of application timing, but due to greater wash-off potential, concentration in soil is greater at an early postemergence application timing (70%) than in a planting green scenario (0 DAT; 55%). When roll-crimping is employed before herbicide application, deposition is reduced by approximately 70% compared with bare ground regardless of application timing. After accounting for wash-off dynamics, total recovery was greater when pyroxasulfone was applied at an early postemergence timing (55%) compared with a planting green scenario (0 DAT; 45%). Inclusion of hairy vetch (Vicia villosa Roth) in mixture with cereal rye further decreased herbicide deposition (85%) into roll-crimped residues at a 5 Mg ha⁻¹ biomass level, but comparatively greater wash-off of pyroxasulfone resulted in similar soil concentration compared with cereal rye monocultures. Our results quantify the relative effects of cover-cropping tactics on initial concentrations of herbicides in soil.